Positron emission tomography (PET) is a nuclear medicine technique which allows us to obtain some anatomic information (e.g., axial, coronal or sagittal images of the brain) as well as dynamic functional data (such as regional cerebral glucose consumption rate). The unique property of PET is that it provides physiologic and pathophysiologic information not available with any other imaging procedure. Using a variety of radiopharmaceuticals (mostly 18F-deoxyglucose and 18F-L-Dopa) as tracers, we have investigated with PET, brain tumors and movement disorders (Parkinson disease, in particular). During the last year we have completed a study on steroids effects on cerebral glucose utilization in patients with brain tumors and found that they reduce the global rate of cerebral glucose consumption.